Ambient vibration tests and real-time health monitoring have become more accepted in civil engineering. Time
domain and frequency domain algorithms have been used in structural identification using the output-only
measurements. The purpose of this paper was to investigate the effectiveness of the data-driven subspace
identification algorithms in modal analysis of bridges from output-only measurements. The stochastic subspace
identification (SSI) algorithm was examined through a numerical truss bridge and a real concrete highway bridge. In
the modal identifications, the simulated dynamic responses of the truss bridge with abrupt damages during the
excitation and the actual acceleration measurements from a real-time health monitoring system were used as the
output data for the numerical truss bridge and the real highway bridge respectively. Stabilization diagrams with a
range of model orders were used to determine the modal frequencies, damping ratios, and mode shapes.
As one of the environmental conditions, temperature fluctuations can have a great effect on the dynamic
characteristics of bridges. It is useful to learn the pattern of changes in frequencies due to temperature fluctuations.
The variation of frequencies with respect to temperature was investigated using one-year ambient vibration data of a
highway bridge. The modal frequencies and temperatures were correlated, which showed that such correlations for
most modes can be represented by single or bilinear lines.
 B. Peeters, C.E. Ventura, Comparative study of modal analysis techniques for bridge dynamic characteristics,
Mechanical Systems and Signal Processing, 17 (2003) 965-988.
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decomposition, in: Proceedings of IMAC 18, the International Modal Analysis Conference, San Antonio, TX,
2000, pp. 625-630.
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Applications, Kluwer Academic Publishers, 1996.
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instrumental variables and subspace fitting, Automatica, 33 (1997) 1603-1616.
 B. Peeters, G. De Roeck, Reference-Based Stochastic Subspace Identification for Output-Only Modal Analysis,
Mechanical Systems and Signal Processing, 13 (1999) 855-878.
 B. Peeters, G. Lowet, H.V.D. Auweraer, J. Leuridan, A new procedure for modal parameter estimation, Sound
and Vibration, 38 (2004) 24-29.
 B. Peeters, G.D. Roeck, One-year monitoring of the Z24-Bridge: environmental effects versus damage events,
Earthquake Engineering & Structural Dynamics, 30 (2001) 149-171.
 J. Chen, J. Li, Simultaneous identification of structural parameters and input time history from output-only
measurements, Comput Mech, 33 (2004) 365-374.
 M.W. Halling, T. Petty, Strong motion instrumentation of I-15 bridge C-846, in, UDOT Report No. UT-01.12,
 T.M. Dye, Forced and ambient vibration testing of a permanently instrumented full-scale bridge, in:
Department of Civil and Envirionmental Engineering, Utah State University, Logan, 2002, pp. 116.
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Conference, Santa Barbara, CA, USA, 1998, pp. 111-116.